1. Field of the Invention
This invention relates to an ignition timing control apparatus for internal combustion engines operating to detect knocking occurring in an internal combustion engine (hereinafter referred to simply as an engine) by the vibrations, sounds or the like generated inside and outside the cylinders of the engine due to the elevated pressure of burning gases in the cylinders of the engine and to adjust the spark time of the engine so that knocking may be kept down within a predetermined magnitude.
2. Description of the Prior Art
It is generally known that there is a close relation between the spark time and the burning gas pressure in each of the combustion chambers of an engine. The pressure in the combustion chamber when a mixture gas burns therein without causing knocking contains no high frequency waves (which are caused by intermittent and rapid combustion of a mixture gas and have frequency components normally ranging from 5 KHz to 10 KHz which belong to a frequency band determined by the bore of the engine cylinder and the sound velocity in the burning gas). However, once knocking has started to appear, such high frequency waves begin to appear approximately at the moment of occurrence of the maximum burning gas pressure in a combustion chamber, which causes vibrations or sounds to be transmitted to the outside of the engine cylinders. Upon studying the state of the generation of internal pressure signals in the engine cylinders or that of the vibrations or sounds transmitted to the outside of the engine cylinders, it is seen that initial knocking (trace knocking) begins to appear from a crank angle of the engine where the burning gas pressure in the combustion chamber becomes maximum, and, as knocking is promoted gradually (to the extent called light or heavy knocking), a greater amount of high frequency waves come to appear preceding the maximum pressure point in the combustion chamber (namely, toward the point of the spark time). Therefore, it is possible to have the efficiency of an engine elevated greatly, if such vibrations or sounds caused by knocking and transmitted to the outside of the engine cylinders can be picked up with high precision and fed back to control the ignition timing of the engine. In doing so, it is necessary to obtain a knocking detector which can detect with high precision the state of knocking, which is an important factor to be fed back, and can operate stably under severe environmental conditions which the engine-mounted vehicle would encounter. If the knocking detector should fall into any abnormal state such as wire breakage, electrical leakage, breakdown or the like, an output signal of the knocking detector would almost always dispappear in such a case, so that the spark time would continue to be advanced even under a knocking condition, due to the misjudgement that no knocking has ever occurred. Under such a condition, heavy knocking would occur, and not only the driving performance of the vehicle would be deteriorated, but also there could arise a disastrous problem such as the destruction of the engine.